Interstitial corrosion is a phenomenon that is well known in the electrochemical field. This occurs when components of electrolytic cells, for example chlor-alkali cells, made of titanium or titanium alloys, come into contact with solutions containing chloride ions.
The phenomenon of interstitial corrosion is a major problem in hydrochloric acid electrolysis cells. Electrolysis of hydrochloric acid is an electrochemical process that is receiving increasing interest, as hydrochloric acid is the typical by-product of all the major industrial processes using chlorine: increase in the production capacities of plants of modern design leads to the formation of large amounts of hydrochloric acid, marketing of which presents significant difficulties. The electrolysis of hydrochloric acid, typically carried out in electrolytic cells with two compartments separated by an ion-exchange membrane, leads to the formation of chlorine in the anode compartment, which can be recycled upstream, resulting in a substantially closed cycle, practically free from environmental impact. The constructional materials of the anode compartment must be able to withstand a highly aggressive environment resulting from the combination of acidity, moist chlorine and anodic polarization; for this purpose, for the constructional materials it is preferable to use valve metals such as titanium, niobium and zirconium, among which titanium, optionally alloyed, is the one most commonly used, for reasons of cost and ease of processing. For example, titanium alloys containing nickel, chromium and small amounts of noble metals such as ruthenium and palladium, such as the AKOT® alloy marketed by Kobe Steel, are widely used. The anodes on which anodic evolution of chlorine takes place consist for example of a substrate of valve metal, such as a titanium alloy, coated with a suitable catalyst capable of lowering the overpotential of the anodic discharge of chlorine, generally consisting of a mixture of titanium and ruthenium oxides. The same type of coating is also used as corrosion protection for some components of the anode compartment not involved in the evolution of chlorine, in particular in the zones that may constitute interstices subject to stagnation of electrolyte. Lack of sufficient exchange of electrolyte may in fact lead to local discontinuity of the passivating layer whose purpose is to protect the valve metal, triggering corrosion phenomena, which are more dangerous the more they are localized on small areas. The external flanges of the two compartments, anode compartment and cathode compartment, which are typically provided with sealing gaskets, are an example of cell zones subject to interstitial corrosion. In the most favourable cases encountered in industrial practice, titanium alloys coated with catalytic formulations based on oxides of ruthenium and titanium can guarantee continuous operation in hydrochloric acid electrolysis plants of between 24 and 48 months, before corrosion problems arise, which lead to loss of seal of the cell elements in the flange zone. The problem of protection of titanium surfaces present as equipment in chlor-soda diaphragm processes is tackled for example in EP0397114. EP0397114 describes the application of a monolayer structure comprising particles of a platinum group metal or an oxide thereof dispersed in a matrix of fluorinated polymer in a volume ratio of 1:2. In the solution proposed in EP0397114, the fluorinated polymer has the function of a binder, in which the electrically conducting particles are dispersed. In this special type of structure, electrical continuity between the catalytic particles is poor. The consequence of the lack of electrical continuity is failure to establish the passivation mechanism and hence protection of the titanium substrate. In the special structure described in EP0397114, moreover, the amount of polymer cannot be reduced as there would be inadequate mechanical stability of the structure itself. In order to improve the competitiveness of industrial processes for hydrochloric acid electrolysis, where the reaction environment is much more aggressive than in chlor-alkali cells, further increase in the useful life of these components becomes necessary.